The black plant looking shapes in these pictures are dendrites. Sometimes referred to as pseudofossils or ferns, they are mineral formations created by water making its way into rock fissures. The water evaporates or flows off and leaves these black (manganese oxide) shapes behind.
The shapes remind me of images I have seen from graphed fractals in mathematics. This rock was found in Louisville, Kentucky (Jefferson County) while looking for fossils in the Silurian and Devonian rock layers. The rock layer is sometimes referred to as the Louisville or Jeffersonville limestone layer.